656 A. J. TIEIE 
4. Mineral composition.—In the field, proportions between 
minerals cannot be determined. Under the microscope they can. 
The only true problems which arise concern the existence in given 
rocks of varying proportions of quartz and feldspar, of quartz and 
calcite, of quartz and glauconite, of calcite and siderite, of calcite 
and glauconite, of siderite and glauconite, or of various analogous 
but rarer combinations. At present, arenaceous limestone is used 
to denote a rock preponderantly calcareous and, may we say, one- 
fourth arenaceous. Calcareous sandstone is employed when such 
percentages are reversed. But in the writer’s experience a number 
of rocks exhibit percentages of minerals close to 50:50. It is 
suggested that such terms as calarenite, sidarenite, glaucarenite 
would prove useful and not uneuphonious names for 50:50 com- 
binations of calcite and quartz, siderite and quartz, glauconite 
and quartz. Limestone-glauconite, limestone-ferrite are examples 
of similarly compounded names for other mineral mixtures in rocks. 
Simple field names suggested on page 655 would thus be used only 
in emergencies. 
The presence of glauconite involves a minor problem. Except 
when calcite is a cement, its presence in small percentage in a 
sandstone would hardly warrant the use of the name calcareous 
sandstone. Glauconite, however, throws some light upon the 
conditions of deposition. It is advocated that glauconitic as an 
adjective be employed even if the percentage in a sandstone or 
limestone be as low as 5 per cent. 
5. Fossil content.—Fossiliferous sandstone, shale, and limestone 
are names already familiar. However, every geologist should 
recognize at sight the various invertebrate phyla and the main 
classes. And many geologists could thus characterize fossil- 
bearing rocks as predominantly graptolitic, coralline, vermicosic, 
pelmatozoic, bryozoan, brachiopodic, molluscan, trilobitic, etc. 
Coquina seems to be a term for pelecypodic limerock. 
ILLUSTRATIVE DESCRIPTIONS OF SEDIMENTS 
With some hesitation, the present discussion is closed with 
illustrative descriptions of hand specimens of Cambrian rocks and 
